1. Field of the Invention
The invention relates in general to timing waveform generators, and more specifically to timing waveform generators suitable for use in controlling thyristors in a power converter.
2. Description of the Prior Art
Polyphase bridge converters constructed of thyristors are used to provide an adjustable source of direct current voltage for the operation of D.C. motors. For example, a dual bridge converter may be used to provide an adjustable voltage source for the D.C. drive motor of a traction elevator system. The elevator application places stringent design parameters on the solid state converter, because elevators are required to operate satisfactorily from a normal power supply, and also from an emergency power supply. The converter is expected to satisfactorily handle the transients which may occur on the normal power supply, and it is also expected to satisfactorily handle the severe notching which is present in the waveforms of most emergency power supplies. The functions which are most likely to be adversely affected by transients and notches in the power supply are the timing waveform function, and the phase monitoring function. The timing waveforms in a converter system must be accurately related to the polyphase source voltage, in order to assure synchronism between the phase controller and the power converter. The phase controller controls the conduction or firing angle of the thyristors, and it insures that each thyristor is fired between predetermined angles or end stops. A prior art approach to reduction of distortion in timing waveforms selects an advanced signal and applies it to a phase shift circuit designed to phase shift the signal back to the desired phase position.
The phase monitoring function must correctly interpret transients or notches for what they are, and not as a loss of phase or improper phase sequence, or the power converter will be unnecessarily shut down.
Thus, it would be desirable to provide new and improved timing waveform and phase monitoring apparatus which will operate satisfactorily from a normal power supply with its power line transients, and also from an emergency power supply which may have severe notches in its waveforms.